Elastomeric seals generally are not suitable for ultra high vacuum (UHV) sealing applications because of the inherent open structure of polymeric chains through which molecular gaseous species can diffuse. The lowest achievable He leak rate through elastomeric seals is typically 10−8 cc/sec. Using metallic seals, He leak rates in the range of 10−9-10−11 cc/sec can be easily obtained. Metallic seals, however, generally require high sealing force (Fs) also referred to as mechanical contact pressure Pmc where Pmc=Fs/As and As is the area of the sealing dam. If Pmc exceeds the yield strength of the sealing flange material, flange surfaces can be brinelled when they compress the metallic seal between them to achieve UHV. If brinelled, the flange surfaces will typically require reconditioning before installing new seals. Therefore, metallic seals have been designed to reduce the force required to compress the seal by optimizing seal cross section.
Metallic seals can also offer a longer seal life compared to elastomeric seals in applications where process chemicals would otherwise degrade an elastomeric material, for example in semiconductor processing applications. Elastomeric seals are attacked by highly reactive radicals such as NF3 and O2 which severely damage the polymeric chain structure thereby limiting the seal life. Metallic seals made from nickel, aluminum, tin, and/or stainless steel, for example, can be used in appropriate environments in which specific alloys are found inert. Because of extremely low leakage characteristics, metallic seals are often used to seal poisonous gases, such as PH3 commonly used in semiconductor processing. Metallic seals of appropriate design can achieve leak rates even lower than welded joints. For example, He can have a higher molecular diffusion rate through weld defects than through a metallic seal due to micro cracks, grain boundaries and/or porosity of the welds.
U.S. Pat. No. 6,409,180 to Spence et al. (“Spence”) discloses a UHV metallic seal design similar to the one shown in FIG. 1. The Spence seal consists of four sections: two beams 1 and 2, a column 3, two diagonal braces 4 and 5, and two sealing dams 6 and 7. The diagonal brace angle is 35 to 55 degrees. An recessed surface ABC between beams 1 and 2 forms variable width column 3 having a minimum width at the center.
When the flanges 9 and 10 compress the seal, the seal height is reduced as the column 3 undergoes stable buckling maintaining the sealing dam surfaces 6 and 7 parallel to the sealing surfaces of flange 9 and 10, as illustrated in FIG. 2.
A number of steps are typically necessary for machining the Spence seals from a hollow tube. For example, and with reference to FIG. 3, one method of making the Spence seal may include:                (i) machining an annular recessed surface ABC with a tool T1 having the desired profile;        (ii) sectioning the individual seals,        (iii) machining the first sealing dam with a second tool T2; and        (iv) machining the second sealing dam.        
An alternative method might include:                (i) machining the annular recessed surface ABC with a first tool T1;        (ii) machining another annular recess with a second tool T2 to form the sealing dams; and        (iii) sectioning individual seals.        
There are several disadvantages of the Spence seal design. For example, the large number of machining steps can increase production cost, and the variable thickness column including the braced section requires a high sealing force (Fs) and Pmc which can cause brinelling of the sealing surfaces of the flanges.